E-H Bond Cleavage Processes in Reactions of Heterometallic Phosphinidene-Bridged MoRe and MoMn Complexes with...

Reactions of complexes [MoMCp(μ-PMes*)(CO)₆] with H₂ and several p-block element (E) hydrides mostly resulted in the cleavage of E-H bonds under mild conditions [M = Re (1a) and Mn (1b); Mes* = 2,4,6-C₆H₂Bu₃]. The reaction with H₂ (ca. 4 atm) proceeded even at 295 K to give the hydrides [MoMCp(μ-H)(μ-PHMes*)(CO)₆]. The same result was obtained in the reactions with H₃SiPh and, for 1a, upon reduction with Na(Hg) followed by protonation of the resulting anion [MoReCp(μ-PHMes*)(CO)₆]^-. The latter reacted with [AuCl{P(p-tol)₃}] to yield the related heterotrimetallic cluster [MoReAuCp(μ-PHMes*)(CO)₆{P(p-tol)₃}]. The reaction of 1a with thiophenol gave the thiolate-bridged complex [MoReCp(μ-PHMes*)(μ-SPh)(CO)₆], which evolved readily to the pentacarbonyl derivative [MoReCp(μ-PHMes*)(μ-SPh)(CO)₅]. In contrast, no P-H bond cleavage was observed in reactions of complexes 1a,b with PHCy₂, which just yielded the substituted derivatives [MoMCp(μ-PMes*)(CO)₅(PHCy₂)]. Reactions with HSnPh₃ again resulted in E-H bond cleavage, but now with the stannyl group terminally bound to M, while 1a reacted with BH₃·PPh₃ to give the hydride-bridged derivatives [MoReCp(μ-H)(μ-PHMes*)(CO)₅(PPh₃)] and [MoReCp(μ-H){μ-P(CH₂CMe₂)C₆H₂Bu₂}(CO)₅(PPh₃)], which follow from hydrogenation, C-H cleavage, and CO/PPh₃ substitution steps. Density functional theory calculations on the PPh-bridged analogue of 1a revealed that hydrogenation likely proceeds through the addition of H₂ to the Mo=P double bond of the complex, followed by rearrangement of the Mo fragment to drive the resulting terminal hydride into a bridging position.